I am making a star wars fan game and I want the light saber to follow an object that is a certain color (from a webcam) and I want it to be able to block bullets and other light sabers etc. So I guess what I am asking is how would I use motion tracker and block bullets?
You could do that using python, probably.
this may help http://derek.simkowiak.net/motion-tracking-with-python/
Id like to see your project though. It sounds very cool! could you attach the .blend?
I will add you as a contact so once I actually get somewhere with it you can see it 
thank you for showing interest in it and for sending me the motion tracking link It is very much appreciated.